The present invention relates in general to register control for multi-color printing and pertains, more particularly, to a closed-loop register control system used in conjunction with a commercial printing press.
There is tremendous interest in the printing industry in being able to automatically maintain register between printing cylinders of commercial printing presses. The problem comes about because colors are laid down separately by printing units which are spaced several feet apart. Therefore, the variations of the machinery, ink and paper are such that these individual color printing units have to be adjusted with relation to each other in order to register the colors on the sheet or web. In addition, throughout a printing run, there are upsets such as occur in, for example, splicing in of a new roll of paper or washing of the press unit, which cause short-term changes in the relative register between colors. There are also errors in the printing press preparation beginning with copy layout, through production of the plates, to the mounting of the plates. These errors are compensated for, on-press, by moving the plate image circumferentially and laterally so that all colors are registered as they are printed on the paper or other substrate.
In gravure printing automatic registration is standard. It is necessary to monitor the individual colors and adjust the cylinders. This adjustment usually takes place between each color unit and the next. Also, the dynamics on the stability of paper varies quite a bit.
There has been a long desire to apply the gravure printing techniques to commercial printing. It is noted that in commercial printing the register is generally more stable once it is running, but the initial obtaining of register can be quite wasteful and costly. The acceptance of the gravure techniques in commercial printing has been hampered by two major factors. First is the cost and second is the need to put registered targets on the printers image.
With respect to targets, in many types of commercial work there is a backbone or a clear area in the back of a magazine or a book where a target can be readily applied. However, it has been estimated that at least 50 percent of commercial work does not have non-printing areas which could be used to carry the target. One might contemplate providing an extra margin or area on the sheet. However, this adds extra cost. The ink and paper necessarily printed adds significantly to the cost. Furthermore, it detracts from the payback of close loop registers.
There are several installations known where the gravure technology of close loop register has been applied to commercial printing. The approach is to print individual targets at each press unit on to the paper and to monitor these targets with photoelectric eyes, controlling the mechanical register adjustments with respect to the position of the colors to the machinery.
There are several problems associated with the above mentioned approach:
1. The sheet-to-sheet register variation printed image is often as much as one row of dots thus causing a simple mark detector to see an appreciable amount of jitter.
2. The printed image does not always fit completely over all of the sheet. That is, the image may be correct in one area and be off in another. This problem of image fit can change dynamically with the moisture in the paper or the ink tack. The register target can be held in register and the image, several inches away, can be constantly changing register.
3. There is not always room to put the targets on the page.
4. The web may physically shift, mostly side to side, and any attempt to track the image by looking at its location with respect to the machinery, produces an error. The individual colored images may be correct but the entire web shifts.
5. Conditions that are mostly in need of register correction are often marginal with respect to contrast. That is, for instance, after a clean-up there may be a great deal of ink on the paper causing it to be rather dark. The other condition that can exist is where the contrast is very light due to momentary cleaning of the ink on the press. These are most critical for register, causing large register variations. However, they are also difficult for the simple photo detectors to resolve the printed mark.
Accordingly, it is an object of the present invention to provide an improved technique for overcoming the aforementioned problems.
Another object of the present invention is to provide an improved close loop register control technique that is substantially superior to prior techniques for obtaining and maintaining register on a printing press or other like process that forms patterns on a substrate.